Manufacture of articles from aqueous rubber dispersions



Aug. 10, 1943.

Filed Aug. 15. 1939 PLASTIC MASS 0R OOAGULANT CONTAINING EXHAUSTER FIG. 2

PLASTIC COATING PLASTIC COAT war PLASTIC COATING LATEX con LATEX PLASTIC MASS STRIP-FED swELLme ARTICLE FLOATING wATaR FIG STEPHEN B. NEILEY EMILE E. HABIB I INVENTORS a, Labc -L. Q \SM ATTORNEY Patented Aug. 10, 1943 MANUFACTURE OF ARTICLES FROM AQUEOUS RUBBER DISPERSION S Stephen B. Neiley, Winchester, and Emile E. Habib, Arlington, Masa, assignors to Dewey and Almy Chemical Company, North Cambridge, Mass.,' a corporation of Massachusetts Application August 15, 1939, Serial No. 290,252

4 Claims.

This invention is concerned with the manufacture of formed shapes from aqueous dispersions of rubber, and is directed towards the solution of certain problems which arise in the rubber dipping art when exhausters, Patent No. 2,172,400, issued to Stephen B. Neiley, September 12, 1939, or coagulants are spread over formers and these are subsequently dipped in the dispersion.

It is an object of the present invention to cause the rapid coagulation of the latex in molding or dipping procedures; to form against the surface of the former one or a number of coagulated layers of latex which may later be dried and vulcanized; to form layers of material thickness; to avoid sags, drips and irregularities in gauge; and to provide a simple and expeditious method by which articles having a regular configuration may be stripped from the formers. These and other objects will become apparent from the? specification.

In the prior practice, it was characteristic of. both coats of coagulating agents and coats of exhauster that the materials, being liquids, re-' sponded immediately to gravitational force and drained down the surface of the mold. Inevitably, therefore, upper areas of the mold were covered with less weight of coagulant or ex-i hauster per unit area than were lower areas and, since both coagulants and exhausters work quan-' titatively, both processes possess the disadvam, tage that it is extremely difiicult to maintain a uniform gauge of the rubber which is deposited upon the form.

In contradistinction to the coagulant or exhausting materials previously used, we employ a; plastic system. Plasticity as distinguished from viscosity implies that, upon the application of force, a plastic system will not move. Movementj starts only when the force upon the plastic system exceeds what is known as its "yield value. 5 As applied to our process, a plastic film will not@ respond to gravity since the absolute value of its adhesion to the mold and the yield value of; the film itself may be made to exceed gravitaj tional pull. It is possible, therefore, to distribute over the surface of the mold a completely unii form film 'of an active substance which will; neither slide nor drip and will produce a uniformly gauged rubber deposit.

Although a number of materials which will impart plasticity to a fluid system are well known, and it is within the scope of the invention to utilize any hydrophilic colloid which is comwhich imparts plasticity, we prefer to use bentonite because of its ready availability, low cost, and because it possesses to an outstanding degree the characteristics that we seek. Dried bentonite swells with great rapidity in water and to a lesser extent in other polar liquids, and in the swollen state is slippery, almost soapy in its action. We

form having a coating of the plastic coagulant or exhauster;

Figure 3 illustrates the dipping of the form of Figure 2 into a latex dispersion;

Figure 4 is a vertical section of the form after the operation of Figure 3;

Figure 5 illustrates the swelling of the plastic mass preparatory to stripping; and

Figure 6 illustrates the stripped rubber article.

Dipping bath In carrying out this invention we preferably include in the aqueous dispersion of the rubber a potential coagulant for the dispersion. The potential coagulant is a double or complex salt formed by the reaction of water-soluble amine compounds upon salts of multi-valent metals.q

Many such. complex salts are known. For, reasons of ready availability and lowcost, we prefer to use the salt disclosed in Reissue Patent No. 19,426 to S. B. Neiley, dated January 15, 1935.

Specifically, in-carrying out the invention, 10

parts of granulated zinc chloride are dissolved in 160 parts of water and to this solution is added 15 parts of 28 Baum commercial aqueous ammonia. This solution is added to commercial concentrated latex containing -65% rubber solids which may be compounded, if desired, with sulphur, activator, and accelerator in approprlate amounts.

Characteristic proportions for a workable mixture are: aqueous dispersion of rubber 1,000 parts,

- potential coagulant solution ll to 33 parts.

patible with the 'exhausters or coagulants and Exhauster 53 parts of 40% solution of formaldehyde and 2 parts of glycerine.

. penetrates. I

Although the exhauster material just described Procedure I I 1 about long, then thelifting valve is opened, jerking the formers clear of the water. The

. stripping is complete and practically instanpresent in the dispersion breaks down releasing coagulative zinc ions wherever the formaldehyde is almost as fluid as water, and is in no sense dry when the formeris dipped in the dispersion, the

presence of'the bentonite suflicient to modify materially the characteristics of the solution. Instead of a viscous system which would exist if no bentonite had been added andwhich according to the laws of viscous systems, would thin down I upon the upper portions of the former and col lost as a drip at the lower portion, we now have The process is illustrated in the accom any- Figurelillustratesthedipl llof the form in abath of plastic material containing a coagulant or 'exhauster. When the form ,is

withdrawn from the bath of plastic material con I taining coagulant or exhauster, it is covered with 'a' wet plastic coating, as illustrated in Figure 2.

The form is'then dipped into the latex dispersion I as shown in Figure 3 to bringabout a deposit of rubber on the plastic coating, as shown in Figure a plastic system which will not move until its yield value has been exceeded. As a result no sagging or slipping occurs on the former. Thus we carry and retain a uniform weightof ex,-

hauster per unit area of former and, since the 'exhauster reacts quantitatively with the excess amine present in the dispersion, causes the for-, mation of a uniform, solidified coating of rubber I upon, the former. A rubber article of uniform gauge throughout is the result.

The stripping of dipped goods from the form has always presented a problem to the manufacturer. It, is a step which has always included rolling, stretching or pulling and, although brushing down the article or blowing it oil. the, form are used for specific articles, manual stripping is still generally practiced today. which the stripping operation imposes are so severe that if the film is not first so thoroughly dried while it is in contact with the form that distortion is negligible, and preferably even vulcanized as well, finger marks and rolled edges will appear in the finished article.-

We have found that if the plasticity imparting material present in the exhauster is a highly hydrophilic substance, typified by bentonite, it will swell if the former with its associated rubber coat is dipped in water. The swelling breaks the adhesion of the rubber to the former and if the former is swished up and down in the water bath a few inches below the surface in such a manner that water is forced in between the former and the rubber, a quick upward jerk will strip the rubber coating completely and leave it floating in the water bath. Water, as a chemical substance, is not essential in the stripping operation The colloids swell sufliciently in polar liquids to make stripping possible. Hence, it is within the scope of this invention to swell the colloid and strip the rubber in any operative polar liquid such as water, alcohol, etc. v

In some cases, particularly with dipped articles of large size, dipping of the former into the rubber dispersion results in sufllcient swelling and softening of the hydrophilic colloid to permit easy stripping of the rubber article from the former. In these cases dipping of the former with its associated rubber coating into a bath of water to facilitate stripping may be dispensed with.

In manufacturing practice, a rack full of rubber coated formers is hung from an air lift and the formers lowered into water. The lift is then operated to oscillate vertically through .a path 4. When form is immersed in a bath or water or other polar liquid, as shown in Figure 5, whereuponthe plastic layer. swells and softens If the form is now swished up and down in the water the article will be pulled off the form and left floatingin the water, as shown in Figure 6.

I We find it not strictly necessary, but advantageous to dry the exhauster slightly before the exhauster coated forms are lowered into thedipping bath; drying increases the stability of the exhauster film to such an extent that the weight of coaguiated latex which the film supports is insuflicient to makejt slip on the former, and in addition the lateral movement due to swelling which loosensthe rubber coating is greater and more positive. .The. exhauster coat should not The strains -may be made g follows:

bedone dry. however, for uneven, or ridged articles may result. It is to prevent drying to too great a degree that we include a hydroscopic material in the exhauster, namely glycerine.

If it is preferred to use coagulants on the mold rather than, exhausters, the coagulant solution Parts Bentonite 14% suspension in water 50 sulphate masons-181320 p 7 Water 23 acetone 10 to 20 Diand tri-valent metallic salts and other recognized coagulant bodies may be substituted for the aluminum sulphate if desired.-

We consider it advantageous to use as an ingredient in the coagulant solution. a material which will function in an analogous manner to the formaldehyde upon the bentonite in the exhauster example given above. In the exhauster composition the formaldehyde exerts a. distinct stiifening action upon the bentonite and prevents it from becoming so, slimy that the rubber deposit will'fall oif the former or slide or fold during the dipping procedure. We may achieve thesame result, when usingcoagulants, by adding to the composition about 10% to 20% of a substance such as acetone or alcohol which by its evaporation sufliciently dehydrates the bentonite to insure that no slimy deposits of bentonite will enter the dip tank. min the case when exhausters are used, the coagulant composition including bentonite should not be bone dry when the formers are dipped in the dispersion. The presence of a small amount of glycerine in the composition assures the presence of suflicient retained moisture so that the bentonite, when in the wash water and stripping bath, may be peptized and swollen to the necessary extent to the rubber article is to be stripped the permit stripping. With these precautions taken,

the stripping of a regularly shaped article from former is as easy when using coamflants as manipulating the former to shake the rubber when using the exhauster method which we prefer.

lhe term rubber as used throughout the specification and claims denotes not only the solids of H erea braziliensis, but natural or artificial substances possessing similar essential characteristics, and the term rubber latex denotes not only the natural or preserved latex of Hevea brazilz'ensis, but dispersions of natural or artificial substances which may be coagulated in essentially a similar manner.

We claim:

1. The method of depositing rubber of uniform thickness upon formers and of stripping the rubber deposit therefrom which comprises forming a plastic mixture of water, an active substance selected from the class of exhausters and rubber latex coagulants, and a hydrophilic colloid, coating the former with the mixture and immersing the coated former in an aqueous dispersion of rubber, swelling the colloid to loosen the rubber deposit and manipulating the former to shake the deposit therefrom.

2. The method of stripping rubber articles from formers which includes coating a former with a water suspension of a hydrophilic colloid, forming a coherent coat of rubber upon the colloidal coating by depositing rubber derived from an aqueous dispersion thereon, immersing the former with its adherent coat of rubber thereon in a polar liquid for a time sufiicient to swell the colloid and loosen the rubber and deposit therefrom.

3. The method of forming articles of uniform thickness from rubber dispersions which comprises coating a former with a plastic aqueous mixture of a hydrophilic plasticity imparting colloid and a substance selected from the class of exhausters and latex coagulants, said mixture having a yield value such that the coating applied does not flow under its own weight, immersing the former with its coating in a plastic condition in a dispersion of rubber to cause a deposit of rubber to form against the plastic coating and stripping the rubber from the former.

'4. The method of forming articles of uniform thicknesslfrom rubber dispersions which comprises coating'a former with a plastic aqueous mixture of a hydrophilic plasticity imparting colloid and a substance selected from the class of exhausters and latex coagulants, said mixture having a yield value such that the coating applied does not flow under its own weight, stifien ing the plastic coating sufliciently to support the weight of the rubber coat to be deposited, immersing the former with its coating in a plastic condition in a dispersion of rubber to cause a deposit of rubber to form against the plastic coating and stripping the rubber from the former.

STEPHEN B. NEILEY.

EMILE E. HABIB. 

